drivers: video: fbdev: atmel_lcdfb.c: Add ability to inverted backlight PWM.

The code has a variable to change the polarity of the PWM backlight control but
it was not being initialized. This patch adds a devicetree entry to set the
variable if required.

Signed-off-by: Michael Welling <mwelling@ieee.org>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>